When the jury at Winchester crown court found Danilo Restivo guilty of murder yesterday it brought to an end an international mystery that has baffled and fascinated observers for almost two decades. It's a tragic story that sometimes seems scripted by a screenwriter: there's a serial killer with a bizarre fetish, an innocent young girl, hints of organised crime and a dodgy priest. The trial in which Restivo was found guilty yesterday was, in many ways, just the tip of the iceberg.

The story began back in 1993 in a forgotten region of Italy called Basilicata, also known as Lucania. It's a region that stretches from the arch to the metatarsal of the Italian boot. Almost entirely mountainous, with poor roads and no tourism, it seems like the land that time forgot. It's so remote that this is where Mussolini chose to send enemies of his regime such as Carlo Levi or the mafioso Calogero Vizzini; this is where the kidnapped John Paul Getty III was held captive back in the 1970s. Until the 1950s, some people were still, literally, living in caves, the famous sassi of Matera. Many of the villages are so remote that, even centuries after they arrived, the people still speak Albanian.

The region's capital city is an ugly place called Potenza. Prone to earthquakes and illegal building, it's now a concrete jungle with a spiral of potholed roads leading to the summit. It was here, on a busy Sunday morning on 12 September 1993, that a 16-year-old girl called Elisa Claps met a young man for a date. She was reluctant to go because he was a strange type: he was older than her and he had, according to gossip, a strange fetish – he used to cut women's hair on the back of buses. He was odd-looking too: he had thick hair, large lips and glasses that magnified his already bulbous eyes. He had told her he had a present for her for passing her retakes. Elisa felt sorry for him and went along.

They met in the Church of the Most Holy Trinity in the city centre at around 11.30am, just as mass was finishing. Elisa was never seen again. Her disappearance seemed inexplicable. There were so many people around and yet she had just vanished.

Suspicion immediately fell on Restivo. He claimed to have seen Elisa leave the church and that he had stayed there to pray. He returned home with a cut on his hand just over an hour later. He said he had fallen on a building site, but the cut was, strangely, to the back of his hand. That afternoon he went to Napoli and the priest of the Most Holy Trinity, Don Mimì Sabia, locked up the church and went off on retreat. Before anyone had realised the seriousness of Elisa's disappearance, two of the protagonists had already left the city.

Conspiracy theories

Ever since that first day, there have been lies and misinformation. "Everything I come across in this case," Gildo Claps, Elisa's oldest brother once said to me, "I've had to smash into a thousand pieces." There were immediately many false sightings. Various sources mentioned seeing Elisa in a white Fiat Uno, either in Potenza or Rome. Attention began to focus on a young man called Eris Gega, an Albanian who was said to have repeatedly lied about his movements that Sunday morning, claiming he had never met Restivo. Restivo, Gega and Elisa's friend Eliana De Cillis were all tried for giving false testimony. Of the three, only Restivo was found guilty.

The case gradually became, for many, an obsession, one of the iconic Italian mysteries that enabled people to engage in dietrologia, literally "behindery" or conspiracy-theorising. Elisa's face – her long, dark hair, thick glasses and carefree smile – haunted the nation. Silvana Ferrazzano was a friend of Elisa's who alleged that Elisa had been bundled into a white Fiat Uno and forced into prostitution. Three people with tenuous links to the case died in car crashes. On one Italian website, Popolo della Rete, there's a discussion forum about the Elisa Claps case with more than 3,000 posts, many of them essay-length monologues containing ideas, hunches, suspicions and accusations. More than a quarter of a million people have read the forum.

Strange, disparate pieces of evidence continued to emerge. The diary of Elisa was analysed and it was discovered that a page has been ripped out. Scientific tests on the few fragments they were able to reconstruct revealed that there were words written in Albanian. The pista albanese remained, for years, one of the best leads. In 1994, a traffic warden was convinced he had seen Elisa in Albania. Police and camera crews went to the country and even found someone who looked extraordinarily similar to Elisa. But there was no trace of Elisa herself.

In the meantime, Gildo had set up a website gathering information and leads regarding his sister's disappearance. One afternoon an email purporting to be from Elisa was sent to it. It claimed she was alive and well in Brasil and didn't want to be contacted. Gildo immediately ascertained that the email had come from an internet cafe in the centre of Potenza. When they rushed round there, they discovered that Restivo had just left the premises.

One of the people who studied the entire case for years, a Catholic priest called Don Marcello Cozzi, wrote recently of the extraordinary fact that there have been more depistaggi, "misleads", than leads: "There have been too many Elisas seen in Italy and around the world," he wrote wearily, "too many coincidences and half-truths and half-lies, too many errors and in the end too many oddities that one feels justified in believing a worrying hypothesis: the existence of a occult but organised production room . . . made up of strong but hidden powers . . . which went into action when it was necessary to intervene to defend someone or hide something."

The Claps family, their private investigator and the production team of Chi l'Ha Visto (the Italian equivalent of Crimewatch) were convinced that there was a concerted coverup. Suspicions were such that the entire investigation into the Elisa Claps case was taken out of local hands and moved to Salerno, some 120km away.

Attention also turned to the strange figure of Don Mimì. Until his death in 2008 he had been a stern, cultured priest, someone who was at the very centre of local power. Don Mimì repeatedly obstructed all investigations: he refused access to his church and his was the only church in Potenza not to ring bells for Elisa on the decade anniversary of her disappearance. Many of his congregation were from the upper echelons of local, even national, politics and a former Italian prime minister. According to Marco Gallo, a private investigator who has worked pro bono for the Claps family for more than a decade, Don Mimì was homosexual. He adds: "This whole story is a disaster. It's a complete, tragic mess. You never touch the bottom. Everywhere you put your nose there's something rotten."

Murder in Bournemouth

A genteel town on the south coast of England, Bournemouth is a far cry from Potenza. Amid the many pensioners are hordes of foreign students coming to improve their English. It seems far removed from the gothic noir of Italian mysteries: in a recent survey, Bournemouth was found to be the happiest place in Britain, with 82% of interviewees expressing contentment with their lot.

But in 2002, the town was to become unexpectedly linked to the Elisa Claps mystery. On 12 November, a seamstress called Heather Barnett was brutally murdered at 211 Capstone Road. She was found dead in her bathroom by her children on their return from school. They called the police and then ran out of the house, understandably in a state of extreme shock. The person who immediately comforted them, who was just arriving home as they ran out, was an Italian who lived in the house opposite, Danilo Restivo.

As the forensics experts went to work on the crime scene, it became obvious that this was a bizarre murder. There had been no forced entry. There were spatter stains on the patio doors of Barnett's sewing room and then long stains of blood going through the flat into the bathroom as if the assailant had dragged the dead or unconscious body. The deceased's bra had been cut between the cups and both breasts had been removed and placed on the floor. Her neck had been cut from ear to ear and strands of hair placed in both of her hands. Her trousers had been opened and lowered and a gloved hand placed inside her knickers, although there was no suggestion of sexual assault.

Using luminol to identify minute traces of blood, forensic investigators were able to ascertain that the murderer then walked back towards the sewing room, the bloodied footprints becoming fainter with each step. But there the footprints stopped: in the hall back to the front door there was no blood, leading them to suspect that the assailant had changed their shoes. The time of death, they estimated, was very soon after the last sighting of Barnett alive, when CCTV footage captured her white Fiat Punto turning into Capstone Road shortly after dropping her children off at school that morning.

Slowly a suspect emerged: in an interview with police the day after his mother's murder, her son Terry said that her keys had gone missing the week before after an Italian, "Danny", had come round to ask her to make some curtains. When police were conducting house-to-house inquiries five days after the Barnett murder and asked Restivo to show them the shoes he was wearing on 12 November, they found his Nike trainers in the bath, soaking in bleach. The trainers were confiscated.

A cunning, clever criminal

And yet, just as in the Elisa Claps case, police appeared unable to pin anything on Restivo. A man who appeared so socially dysfunctional, almost a little simple, was also, it seemed, a lucid, clever criminal, able to plan and execute murders without leaving any incriminating evidence. One of the original policemen who interviewed him in Italy after the disappearance of Elisa Claps recalled a man who was "prepared, cool, very cunning, precise in his answers".

What emerged when Dorset police put Restivo under surveillance was acutely worrying: Restivo had the habit of going to a secluded park outside Bournemouth. He was filmed watching single women, sometimes ducking into the long grass as they walked past. Even though it was spring he was seen wearing gloves. He would take off one shirt only to put on an identical one stashed in the boot of his car. He changed his trainers. On one occasion, coincidentally the 12th of the month, the same date that both Heather had been murdered and Elisa went missing, police were so concerned that they called in uniformed officers to search Restivo and his car. They found a filleting knife, two pairs of scissors, a balaclava and gloves. There was nothing illegal, as such, in those possessions, but common sense suggested that here was a man on the brink of committing another horrific crime.

As well as cunning and forensically clever, Restivo was also a man who was used to being molly-coddled, who made a habit of exploiting his childish, dysfunctional side to persuade women to care for him. He originally moved to Bournemouth in 2002 because he had met a woman online. Fiamma Marsango was a large, expat Italian suffering from arthritis. She had two sons from a previous marriage. Covert police recordings from their home revealed a couple in which the older woman looked after an almost babyish man: she chided him for his lies and tried to coax him into learning English. Restivo even appeared to think he was a child. He mused about "my wonderful innocence" and talked to his parents in Italy as if he were still a young boy.

Hair fetish

Now aware of Restivo's hair fetish, the police were gathering witnesses from Bournemouth who had had their hair cut. Two schoolgirls reported having their hair snipped when Restivo was sitting directly behind them on the bus. One of them described finding something white and sticky in her hair afterwards. Another man described seeing Restivo sitting behind a woman on a bus with her hair, and his hands, under a jacket placed on his lap. The inference was, of course, that he was masturbating. In one of the covert recordings from Restivo's home, he was heard talking of his love of hair: "[when I] touch the hair, hold them in the hand . . . then everything is visible, everything."

It emerged that Restivo also had a long track record of sadism. When only 14 he had tied up two young boys in the courtyard outside the city library where his father was director and tortured them, inflicting cuts with a small knife. Their families dropped charges against Restivo in return for the sum of 1m lire. He had also spied on young students living opposite him, phoning them to describe his excitement at their clothing or their movements. In many ways he was the archetypal serial killer, moving from the trauma of a botched operation on his tonsils to the infliction of wounds on others and, finally, to murder.

And yet, despite all that behavioural evidence, there was no smoking gun or bloodied blade. Police appealed twice on Crimewatch for witnesses to come forward. Forensic evidence had revealed that the lock of hair in Heather Barnett's left hand had been cut from her own head, but that the hair in her right hand wasn't hers. Extensive examination of that mystery lock revealed that the person from whose hair it had been cut had been to Florida and Spain or southern France shortly before. They had changed diets. Police were desperate to trace the person in the hope of linking her to Restivo but, despite the appeals on Crimewatch and extensive trips to Italy to take DNA samples from other women, they never found the owner of that mystery lock.

Evidence against Restivo was, however, slowly stacking up. When his flat was searched in 2004 a lock of hair tied with green cotton was found in a Tesco plastic bag under a chest of drawers. The trainers he had soaked in bleach had revealed minute traces of blood on the inside as if a bloodied sock had been placed inside them. The computer he claimed to have been working on at an educational centre on the morning of Heather Barnett's murder had been subject to an "evidential capture" by a digital forensics expert and it was shown that there was no user activity between 9.08am and 10.10am. His alibi, as in Italy, suddenly appeared very shaky. But most tellingly of all, a green towel found in Heather Barnett's flat revealed a DNA profile that was compatible with Restivo's: the chances of that DNA coming from someone other than Restivo were 57,000 to one. When confronted with the evidence from the towel, Restivo suddenly claimed – something he had never mentioned before – that he had taken it round to Barnett's house to get a match on the curtains he wanted to commission.

Discovery of Elisa Claps's body

The case against Restivo was reaching a critical mass when, on 17 March 2010, an extraordinary discovery took place back in Potenza. Elisa's body was, incredibly, found in the very church where she had gone missing 17 years before. Throughout Italy there was disbelief and indignation that she had been there all along. Workmen had been trying to fix a leaking roof in the church and, having gone up seven flights of stairs, they discovered, in a tiny, cramped garret beside the bell-tower, the mummified, skeletonised body covered by a few tiles.

The contrast with the familiar, smiling shot of Elisa that had been used to publicise her disappearance couldn't have been more stark: there were dark stains against the wall where her pelvis and chest would have been, though now they had shrunk and disappeared and all that was left was a black memory of her form on the wall. There were two shoes at odd angles with barely any legs in them, a jumper that looked more like a brown string vest, a head visible with some teeth but disconnected from the spinal cord. Elisa had been reduced to dust. Half-covered with debris were her perfectly folded glasses. It was a terrible, moving image, a reminder that a 16-year-old girl with her life before her had been left to rot, to sink into dust all alone.

It wasn't just Elisa's family that was outraged. Italy in general and the city of Potenza in particular were bewildered. Whether because of incompetence or corruption, the church had never even been searched thoroughly. It beggared belief: a family had lived with tragic uncertainty for 17 years; a city had searched in sidestreets and its soul in vain. There was a groundswell of fury and outside the now-impounded church a spontaneous "garden of Elisa" grew up with flowers and teddy bears and indignant messages demanding justice.

The forensic examinations on Elisa's remains were painstaking. Her clothing and skeleton revealed many insights into the nature of her death: cuts to her bones suggested that she had received at least nine stab wounds from behind, largely to her ribs, and three from in front. One, going through the front of her neck to her spinal cord, might have been made with a pair of scissors. Her hands, rehydrated to reveal cuts, demonstrated classic "defence wounds". Her trousers, top, bra and knickers had been cut with scissors and her trousers unzipped and lowered. There were the dessicated remains of a single strand of hair, "pelliferous" material, in each hand. Traces of haemorrhages to the inner thighs and mammaries suggested the attack may have contained a sexual element. Most tellingly, DNA matching the DNA profile of Restivo had been found, although the source of it – saliva or blood – was uncertain. A red button had been discovered underneath Elisa's body. It measured 13mm. It was exactly the same dimension and colour as the buttons of ecclesiastical cassocks. It wasn't long before a photograph emerged of Don Mimì with a button missing from his cassock.

The arrest of Restivo

Within weeks of the discovery of Elisa Claps, Restivo was arrested in Bournemouth, accused of the murder of Heather Barnett. The discovery of Elisa was the last piece of evidence the British police needed. The coincidences were too great for common sense to ignore: both women, on different sides of the continent, had connections with Restivo. Both had been found with severed clothing and lowered trousers, their hair and their throats cut. Hair had been found in both of their hands. When a judge, in a ruling before the start of the trial in Winchester, allowed the introduction of the Italian evidence because of its "striking similarity", it finally looked as if the police had a convincing case against the "Barber of Potenza".

Restivo's conviction yesterday for Heather Barnett's murder brings to a close an extraordinary story, but many mysteries remain. Some observers are convinced that a compulsive, psychotic killer such as Restivo must have commited more murders between 1993 and 2002. Over the years he has been linked to many other deaths, particularly to a series of brutal murders in southern France and Spain. In September 1997 a young French-Algerian woman from Perpignan, Moktharia Chaib, was stabbed and her breasts, as well as other body parts, removed. Marie Hélène Gonzalez had, in 1998, been brutally mutilated, having disappeared in Perpignan. In 1999, in Puerto de Alcuida, Majorca, a British woman called Yvonne O'Brien was stabbed 40 times and one of her breasts was removed. On Easter Day 2003 a South Korean woman adopted by an Italian family, Erika Ansermin, disappeared. Her body has never been found but a photograph of her, downloaded from an Italian news channel, was found on Restivo's computer.

The links to those murders may be fanciful, but there's one murder in the same suburb of Bournemouth that many are convinced was commited by Restivo. In July 2002, a young South Korean girl, Jong-Ok Shin, known as Oki, was walking home late at night. She was stabbed and left for dead. She died shortly afterwards, having described a man in a "mask". Given her limited English, some have assumed she meant a balaclava. There are unconfirmed reports that her hair was also cut. The connections to Restivo are tenuous, but suggestive: the murder happened only three blocks from his house; it took place – like the murders of Elisa and Heather – on the 12th of the month. A man called Omar Benguit was tried an extraordinary three times for the murder (because of two mistrials). His eventual conviction for the murder appears to many, including professional criminologists, a tragic miscarriage of justice.

Italian church coverup?

But it is, of course, in Italy that most mysteries remain. There is growing evidence that Elisa's body was found two months before the official discovery in March last year and that the official discovery was a mise-en-scène. According to one of the interim priests in the Church of the Most Holy Trinity, two cleaners discovered the body in January 2010 but were persuaded to keep quiet while the Catholic church, aware of the furore that would result, fretted about what to do. The leaking roof was, allegedly, a pretext to send workmen up to the garret two months later. In fact, workmen had been in that same garret back in the mid 1990s and had seen only a huge pile of tiles and rubble. The fact that Elisa's body was found as it was, exposed and with her glasses perfectly folded at her feet, suggested to many that the crime scene had been tampered with, that tiles and rubble had been removed in a deliberate attempt to expose the corpse. And even though the famous red button from the scene was found not to match the others from Don Mimì's cassock, the role of the late priest is still highly suspicious. He claimed never to have known Danilo Restivo, but photographs have since emerged of him attending the man's 18th birthday party. Restivo was a regular in his church. Why would he lie about something so innocent? And what lengths would he – an eminently blackmailable man – have gone to hide the truth?

Restivo will now be extradited to Italy and tried for the murder of Elisa Claps. But in many ways the story is now less a whodunit than a who-covered-up. Having fought against the pomposities and silences of the Italian elite for almost two decades, the courageous Claps family are now demanding that not just Restivo, but also his accomplices, should face the music. "Because," Gildo said to me recently, "if this is allowed to happen, if with the right connections, with the right power and complicities, you can remain unpunished despite murder, if this is allowed to happen, it means this country is definitively compromised. It's as if this story of Elisa is a way for the country to redeem itself. Because if it's not possible to obtain justice for an innocent 16-year-old girl who goes missing in a church, well, it means there really is no hope for this country."

Now, almost 18 years since she first went missing and over a year since her mummified body was found, Elisa is finally being buried. On 1 July her body will be released, and her funeral will take place on 2 July in the open air in Potenza. "She's never going back inside a church," her determined mother told me a few weeks ago. Perhaps only when she's finally been laid to rest will her familiar, carefree face no longer haunt the Italian conscience.

Tobias Jones's book about the Elisa Claps mystery, Blood on the Altar, will be published by Faber & Faber in the spring of 2012. www.tobias-jones.com